Having an abortion does not increase a woman's chance of developing mental health problems, according to a large study that challenges anti-abortion groups' claims that termination causes trauma and depression.

The research, commissioned by the Academy of Medical Royal Colleges and funded by the Department of Health, should reassure women that they are at no greater risk than if they give birth, the authors said.

The biggest study worldwide of the relationship between termination and mental wellbeing is published and concludes: "The best current evidence suggests that it makes no difference to a woman's mental health whether she chooses to have an abortion or to continue with the pregnancy."

The research undertaken by the National Collaborating Centre for Mental Health (NCCMH) at the Royal College of Psychiatrists assessed 44 studies from 1990-2011 which examined data on hundreds of thousands of women at least 90 days after an abortion.

The researchers found that an unwanted pregnancy does involve a heightened risk of mental health problems, but added that the rates were no different whether they had an abortion or give birth.

Professor Tim Kendall, director of the NCCMH, said that about a third of women who have an unwanted pregnancy suffered depression and anxiety compared to 11% to 12% of the general population.

An unwanted pregnancy may cause mental health problems, a woman may already have problems before becoming pregnant, or it could be a combination of the two, he added.

The review had looked at only the mental health aspects of abortion, not the ethics of abortion or its physical consequences, Kendall added. Research should now concentrate on helping women cope with the impact of an unwanted pregnancy.

Women with a history of mental problems were more likely to experience problems after a termination, and other factors such as being pressured by a partner to have a termination could also increase the chance, the researchers found.

Tracey McNeill, the vice-president of Marie Stopes, which performs 66,000 abortions a year in Britain, said the review reinforced its belief that having an abortion has no greater effect on a woman's mental health than continuing with a pregnancy.

She said: "In our experience, for every extra week a woman carries an unwanted pregnancy, it can represent an extra week of distress. This is reinforced by the review's finding that while abortion does not have a disproportionate impact on mental health, having an unwanted pregnancy does.

"This coincides with our strong belief that all women should be able to access non-directive counselling, then be able to exercise their right to be referred for an abortion without delay, if this is the option they have chosen."

Dr Kate Guthrie of the Royal College of Obstetricians and Gynaecologists said it had revised its guidelines to take account of these findings. "The recommendations highlight the need for service providers to inform women about the range of emotional responses that may be experienced during and following an abortion."

Anne Milton, the public health minister, said the findings of "this important review" would be considered when the Department of Health updates its sexual health strategy next year. "What is clear is that having an unwanted pregnancy has implications for people's mental health and wellbeing", she added.

Anthony Ozimic, of the anti-abortion Society for the Protection of Unborn Children, dismissed the study's findings as predictable, accused its authors of ignoring key studies and said evidence showed that abortion involved an increased risk of depression and post-traumatic stress.

"Clinical case studies and stories written and told by many women confirm empirical findings of the psychological harms of abortion," he said.

"Prior mental health may influence mental health after abortion, but does not begin to account for all of the effect. Abortion is associated with severe negative psychological complications for some women.

"Women experienced feelings such as shame, guilt, grief and regret after an abortion."